The invention relates to a fast scanning voltage ramp generator and a streak camera using such a generator.
As described in xe2x80x9cImprovement in avalanchexe2x80x94transistor sweep circuitry for electropic streak camerasxe2x80x9d by S. W. Thomas, R. L. Griffith and W. R. McDonald (16th International Congress. xe2x80x9cHigh Speed Photography and Photonicsxe2x80x9d. Strasbourg, Aug. 27-31, 1984) streak cameras used for the diagnostic of fast photonic phenomena (in visible light or X-rays) are composed of a photocathode that converts photons into electrons, an electron acceleration and deviation system, and an image recovery system.
FIG. 1 diagrammatically illustrates such a streak camera that includes:
a photocathode 10,
a horizontal slit 11,
two deflection plates 12,
an anode 13,
an image recovery system 14.
In this type of camera, the basic phenomenon as a function of time is spread on the recovery screen 14. It is transformed into a function of a distance. The use of such a camera requires the use of a scanning generator that outputs a positive ramp applied to one of the deviation plates 12 of the electron beam, +V1, and a negative ramp applied to the other deviation plates 12, xe2x88x92V2. The result is a common mode about the average potential of these two plates 12.
The quality of the resulting final images is directly related to the quality of the scanning ramps:
linearity,
speed (in volts/nanosecond),
stability.
For the technology used to make this type of scanning ramp generator, the cameras may be classified as a function of the speed of the phenomena to be observed.
Thus, we refer to:
xe2x80x9cmicrosecond cameraxe2x80x9d: scanning period greater than 1 xcexcs
xe2x80x9cnanosecond cameraxe2x80x9d: scanning period less than 1 xcexcs
xe2x80x9cpicosecond cameraxe2x80x9d: scanning period less than 1 ns
xe2x80x9cfemtosecond cameraxe2x80x9d: scanning period less than 100 ps.
A voltage of about 400 volts per deflection plate 12 (or per ramp) is necessary to obtain a spread (distance AB illustrated in FIG. 1) equal to 30 mm.
The following components must be used to obtain such a voltage:
400 Vxcexcs (xe2x80x9cmicrosecond cameraxe2x80x9d): triode type vacuum tubes MOSFET type transistors,
400 V/ns (xe2x80x9cnanosecond cameraxe2x80x9d): avalanche type transistors installed in a chain,
400 V/100 ps (xe2x80x9cpicosecond cameraxe2x80x9d): avalanche type transistors installed in a chain. But in this case a high voltage is generated (for example 3 kVolts in 1 ns), and only a linear part of about 400 volts is used.
The combination of avalanche transistors in the form of a chain is thus described in xe2x80x9cAvalanche transistors provides fast power transientsxe2x80x9d (Electronic engineering, February 1991).
All ramp generators thus obtained according to known art are usually complicated, close to the limits of the technology and furthermore very frequently have a high impedance output. This is applicable for generators described in xe2x80x9cThe development of high peak power solid state pulse generatorsxe2x80x9d by A. K. L. Dymok-Bradshaw, J. D. Hares. P. A. Kellet and J. Westlake (Kentech Company, 26.5.1994), which considers the use of chains of avalanche transistors for an application to high speed optical cameras.
The purpose of this invention is to overcome the disadvantages of devices according to known art by proposing a generator outputting a fast positive ramp and/or a fast negative ramp using the principle of chains of avalanche transistors.
This invention relates to a fast scanning voltage ramp generator including at least one chain of N two-pole avalanche transistors in series and a set of N power supply elements in series, where N is an integer number xe2x89xa72, for example equal to 8, the emitter and the base of each transistor being connected to each other and also being connected to the collector of the next transistor in the chain, an input circuit generating a voltage pulse connected to one of the two transistors at the end of the chain, characterized in hat the set of N+1 power supply elements in series is formed by a set of N+1 zener diodes in series between a high voltage power supply and the ground, and in that the collector and the emitter of each transistor in the chain are connected firstly to the cathode and to the anode of a corresponding zener diode through at least one resistance, and secondly to the ground through a corresponding capacitor.
Advantageously, the generator includes two chains of two-pole transistor to generate two positive and negative voltage ramps.
Advantageously, the input circuit in the first chain is connected to the first transistor in this chain through an inverter transformer and a capacitor so as to transmit a positive pulse received at the input on the collector of this first transistor. The emitter of the last transistor of the first chain is connected to the output of the positive ramp through a capacitor. The input circuit to the second chain is directly connected to the emitter of the last transistor in the second chain to send it a negative pulse received as input. The collector of the first transistor in the second chain is connected to the output of the negative ramp through a capacitor. A resistance is placed between the high voltage power supply and the zener diodes in series.
Advantageously each resistance used in each chain is composed of three surface mounted component type resistances.
Advantageously, each capacitor in each of the two chains is of the xe2x80x9cpatchxe2x80x9d type.
Advantageously, the avalanche transistors are of the FMMT 415 type made by the Zetex Company.
The invention also relates to a streak camera using a ramp generator as described above.
The ramp generator according to the invention using a particular topology of printed circuit and particular avalanche components enables:
a very small size ( less than 50 cm3),
a single power supply (about +2500 volts),
an amplitude of about 1000 volts on 50 ohms for each output,
a slope of 10 volts/picosecond, namely 400 volts in 40 picoseconds (application to femtosecond cameras),
stability when starting (jitter) of 5 picoseconds.
ZTX 415 or FMMT 415 avalanche type transistors made by the ZETEX Company as used normally and as indicated by the manufacturer cannot switch in less than 1.5 ns; in the generator according to the invention, they are used to obtain ramps of 0.1 to 0.13 ns.